I am not a pro player nor do I play ranked games as of now. This is just how I play Sona and what I've learned from playing opposing Sona players.

This guide is basically taken from my experience exploring what Sona had to offer as a champion. I've played her these ways and will talk about what each build entails as well as when to use what build, because there is no set way to playing a champion. You don't know how a game is going to go. So everything here is up for change and situational.

Obviously with the v1.0.0.124

Sona

Aria of Perseverance
Base heal reduced to 25/50/75/100/125 from 35/70/105/140/175
Ability power ratio reduced to .25 from .35
Now grants double her passive aura bonus to her heal target and herself for 3 seconds
Reduced the lockout time while casting Sona's songs to make them more responsive
Fixed a bug where Hymn of Valor and Aria of Perseverance caused her to deal 20% reduced damage:

This kills me because her AP ratios were already horrible. I haven't played her since this patch came out. I'll update this guide once I do play with what I need can be done.

She is still a viable support in game, but I feel like we might see an increase in other supports like Janna.

I picked up Sona around level 20 for a couple of reasons:
1) My friends were always dying for some reason (We're just having fun)
2) They were definitely better AP/AD/Tanks than me. <--- so where did that leave me?

The answer was:

SUPPORT TIME!

I didn't choose Soraka, because honestly, she kind of freaks me out. Taric has a great stun, but I just wasn't down for a melee champion. Janna was ridiculously good so I will probably try her out in the future. The bottom line was: I wanted a champion who could heal and have some type of AOE (area of effect), preferably a stun. Also, I just so happened to have that many IP points to unlock Sona.

Without further ado, I present the 3 sides (The Trinity) of Sona: AP, AD, and Support.

Usually Solo Queue

Honestly I have no idea why I picked AP Sona to be The Father, but it just sort of went with the team. Right away I want to point out, I don't believe AP Sona is really viable for pro/ranked games simply because you aren't fulfilling the role she's natural at, which is support. However that doesn't mean you can't have fun by kicking a** with AP Sona. You will get trolled a bit, but not as bad as if you go AD Sona

Pretty Standard Runes for any Caster.
Greater Mark of Magic Penetration x9
Greater Seal of Replenishment x9 Sona is so incredibly mana hungry early game which is why as an AP player, you definitely want to pick up mana regen.
Greater Glyph of Cooldown Reduction x9 This allows your skills to be more spammable. Obviously if you want you can always switch these runes out for Greater Glyph of Ability Power x9 or Greater Glyph of Scaling Ability Power x9 for more Ability Power. If you choose to do that, I'd recommend you pick up more CDR items.
Greater Quintessence of Ability Power x3 for the AP or Greater Quintessence of Magic Penetration x3 are both great choices.

Not much to say about this expect to pick up Utility Mastery because as AP Sona, I find myself carrying a lot of the time and you want your buffs to last.

Other than that, the standard 9/0/21.

Now there is room for changes! I've included OPTIONS/ALTERNATIVES. I usually end a game with about 4 items completely built if the game drags on long enough. Items depend on the situation you are in. There is no clear-cut formula to follow. I usually end the game with: Tear of the Goddess, Needlessly Large Rod, Ionian Boots of Lucidity, Morello's Evil Tome, and Glacial Shroud.

Starting Items:

Meki Pendant + Health Potion <--- If you're laning alone or take Mid. This happens. Some people don't like mid or they are already pre-made. (I'll go into more detail about laning later) I find Sona way better with a partner, but you know that it doesn't always work out.

Alternatives:
Faerie Charm + 2x Health Potion (or Mana Potion) + Sight Ward <--- Generally the best start for laning with a partner. You have a ward to prevent ganks and health pots to save mana and heal yourself for sustainability in lane as well as Mana Regen. This is what I call the support starting kit. (So definitely get this is you're going support Sona)

Alternative #2:
Doran's Ring <--- I've seen many people start this way, but think about it mana regen > more mana (I suggest taking this if you go mid)

*If you're doing well, i.e. no dying or being out laned:
I like to get Needlessly Large Rod on my first trip back to base + Sight Ward or Vision Ward. Sona has horrible AP ratios which is why people don't like AP Sona; however, early AP pow wow when you're doing well really makes a difference. I played against a Sona who bought Needlessly Large Rod and out laned and killed us all very quickly.

*If you're doing not so hot:
Boots + Tear of the Goddess or Eleisa's Miracle + wards for survivability. I'd skip the Needlessly Large Rod all together.

Mid Game Items:

*If you're doing well:
Tear of the Goddess + Needlessly Large Rod + Ionian Boots of Lucidity (For the CDR) + Chain Vest or Fiendish Codex<--- This will give you the AP needed as well as CDR and a bit of defense. Since, if you're playing well, the other team starts to focus you.

*If you're doing not so hot:
Tear of the Goddess or Eleisa's Miracle + Ionian Boots of Lucidity + Aegis of the Legion <--- taking on a more support role here + survivability. You don't want to be the feeder on your team. Just make sure your tank isn't getting Aegis of the Legion as well. I'm not sure if it stacks. I don't think it does, so it would be a waste. If your tank gets Aegis of the Legion then go for Frozen Heart

Late Game Items:

*If you're doing well:
Tear of the Goddess + Ionian Boots of Lucidity + Morello's Evil Tome + Rabadon's Deathcap <--- tons of AP, Mana, and CDR.

*If you're doing not so hot:
Tear of the Goddess or Eleisa's Miracle + Ionian Botts of Lucidity + Aegis of the Legion + Glacial Shroud + Banshee's Veil <--- survivability and doing your job as a support.

Alternative Items:

Morello's Evil Tome ---> Mejai's Soulstealer You'll get AP for Assists in the beginning and then for Kills. However, I'm pretty leery about snowball items so I avoid them. But this is an amazing alternative. Played on game with it and did pretty awesome.
Morello's Evil Tome ---> Will of the AncientsGreat if you have another caster on your team that's carrying you guys/ I recommend this item completely. I choose Morello's Evil Tome simply for the CDR and I find that the other AP Caster usually gets to Will of the Ancients before me. I switch between the two when needed.
Rabadon's Deathcap ---> Zhonya's Hourglass <--- depends on the game and what you feel like
Rabadon's Deathcap ---> Archangel's Staff <--- Great item and I find myself usually buying this when I'm not doing great nor horribly.
Banshee's Veil ---> Randuin's Omen <--- Survivablity and slowing people!!

Items to Consider:

Lich Bane: tons of AP damage.
Void Staff: I generally leave this for the real AP casters
Rod of Ages: The health is nice but there are better items to put on Sona